Consumers in the United States will no longer face a backlog when purchasing an iPhone 6 or 6 Plus from Apple's online store, as supplies of the popular smartphones appear to have reached parity with demand nearly four months after their release.

All but one configuration are now listed as "in stock," and spot checks at Apple retail outlets in the southern U.S. show most models available for immediate in-store pickup. The lone holdout— the sim-free iPhone 6 Plus —comes with a lead time of just one day.

The iPhone 6 series has proven to be Apple's most popular iPhone lineup ever, with at least one analysis suggesting that Apple may have sold as many as 69 million of the devices over the holidays. As a result, Apple has struggled to keep up with demand.

Shipping times slipped to as long as 4 weeks for the larger iPhone 6 Plus just one day after pre-orders went live last September, while the 4.7-inch iPhone 6 say waits of 7 to 10 days. Those queues grew shorter in December, though the never dropped below one day.

Supply also appears to be improving in Europe, though Apple's hottest market— China— remains underserved. The company still requires buyers to pre-register online in order to reserve a device for purchase, and recent on-site checks at Apple retail outlets in Hong Kong showed dozens of customers lined up to purchase a new handset throughout the day.